The present invention relates to an imaging lens for forming an image of an object on an imaging element such as a CCD sensor and a CMOS sensor. In particular, the present invention relates to an imaging lens suitable for mounting in a relatively small camera such as a camera mounted in a portable device including a cellular phone and a portable information terminal, a digital still camera, a security camera, a vehicle onboard camera, and a network camera.
In these years, in place of cellular phones that are intended mainly for making phone calls, so-called “smartphones” have been more widely used, i.e., cellular phones with such functions as those of portable information terminals (PDA) and/or personal computers. Since the smartphones generally have more functions than those of the cellular phones, it is possible to use images taken by a camera thereof in various applications. For example, while it is possible to use the smartphones for printing and enjoying images taken, it is also possible to use images in other usage such as processing images to use for game characters or for makeup simulations, dress fitting simulations, and the others. The ways of the image usage were not conventionally common, however, it becomes more common mainly among young people.
Generally speaking, a product group of cellular phones and smartphones is often composed of products with various specifications including from the ones for beginners to the ones for advanced users. Among them, an imaging lens to be mounted in the cellular phone or the smartphone, which is developed for advanced users, is required to have a high resolution lens configuration so as to be also applicable to a high pixel count imaging element of these days. However, as the imaging lens to be mounted in smartphones used for the above-described usages, it is critical to have a small size with a wide angle of view, that is, a wide angle, rather than having a high resolution. Especially in these days, with downsizing and high functionality of smartphones, there are demands for an imaging lens having even smaller size and wider angle.
However, it is also true that even products for beginners are required to have some high resolution. In case of a lens configuration composed of six lenses, since the number of lenses that compose an imaging lens is many, although it is somewhat disadvantageous for downsizing of the imaging lens, since there is high flexibility upon designing, it has potential of attaining satisfactory aberration correction and downsizing in a balanced manner. For example, as a lens configuration composed of six lenses, an imaging lens described in Patent Reference is known.
The imaging lens described in Patent Reference includes a first lens that is negative and has a shape of a meniscus lens directing a convex surface thereof to an object side; a bonded lens composed of two lenses, positive and negative lenses; and a positive fourth lens; a bonded lens composed of two lenses, positive and negative lenses, arranged. According to the imaging lens of Patent Reference, satisfying conditional expression related to curvature radii of an object-side surface and an image plane-side surface of the first lens and a conditional expression related to the two bonded lenses, respectively, it is achievable to satisfactorily correct a distortion and a chromatic aberration.    Patent Reference Japanese Patent Application Publication No. 2011-145315
However, in case of the imaging lens of Patent Reference, since a distance from an object-side surface of the first lens to an image plane of an imaging element is long, it is necessary to bend an optical path by arranging a prism or a mirror between an imaging lens and an image plane, in order to mount the imaging lens in a small-sized camera such as a cellular phone or a smartphone. High functionality and downsizing of cellular phones and smartphones have been advanced every year, and the level of downsizing required for an imaging lens is even higher than before. With the lens configuration described in Patent Reference, it is difficult to attain satisfactory aberration correction while attaining downsizing of the imaging lens so as to meet the demands.
Here, such challenge is not a problem specific to the imaging lens to be mounted in cellular phones and smartphones, and rather, it is a common problem even for an imaging lens to be mounted in a relatively small camera such as digital still cameras, portable information terminals, security cameras, vehicle onboard cameras, and network cameras.
In view of the above-described problems in the conventional techniques, an object of the invention is to provide an imaging lens that can satisfactorily correct aberrations. A further object of the invention is to provide an imaging lens that can attain both downsizing of the imaging lens and satisfactory aberration correction.
Further objects and advantages of the invention will be apparent from the following description of the invention.